1. Field of the Invention
This invention relates to a process for manufacturing organic silazane polymers which are suitably used as precursors for ceramic materials and also to a process for manufacturing ceramics from the organic silazane polymers.
2. Description of the Prior Art
Great interest has been currently shown in ceramics as materials which are excellent in heat resistance, abrasion resistance, high-temperature strength and the like. However, because of the hardness and brittleness, ceramics are very difficult to process. For the manufacture of shaped ceramic articles, it is accordingly employed a method which comprises previously molding a fine powder of ceramic material into a desired form such as by compression, followed by sintering, or a precursor method in which an organic polymer, serving as a preceramic material, is melted or dissolved in a solvent, followed by fabrication into a desired form, and sintering to render the polymer inorganic. The prominent feature of the precursor method resides in that ceramic products of such complex forms as will never be obtained in the sintering method for fine powder can be obtained, i.e. products of specific forms such as fibers or sheets can be manufactured.
Among those materials generally referred to as ceramics, SiC and Si.sub.3 N.sub.4 have attracted generally considerable attention because of the good characteristics thereof at high temperatures, e.g. SiC has a high heat resistance and a high-temperature strength and Si.sub.3 N.sub.4 has a high thermal shock resistance and a high fracture toughness. Accordingly, there have been made various proposals on processes of producing SiC-Si.sub.3 N.sub.4 ceramics and also on processes of producing organic silicon precursors according to the precursor method as is particularly shown (1) to (5) below. However, these proposed processes have still problems set forth below.
(1) In U.S. Pat. No. 3,853,567, there is disclosed a process of obtaining SiC-Si.sub.3 N.sub.4 ceramics in which chlorosilanes and amines are reacted and subsequently heated at high temperatures to obtain carbosilazanes, followed by subjecting them to spinning and infusibilization and then sintering at high temperatures of from 800.degree. to 2000.degree. C. However, this process requires high temperatures of from 520.degree. to 650.degree. C. in order to obtain the carbosilazane, thus being very difficult to apply as an industrial process. In addition, the carbosilazanes are disadvantageous in that the yield of ceramic materials therefrom is as low as about 55%. Examples of this U.S. Patent specification used show only methyltrichlorosilane and dimethyldichlorosilane as the chlorosilanes and methylamine as the amine.
(2) U.S. Pat. No. 4,097,294 describes conversion of various silicon-containing polymers into ceramic materials by pyrolysis. However, it discloses only one example for the silazane polymer and the ceramic yield is as low as 12% in a maximum. Although this United States patent specification describes that ceramic materials may be formed into fibers or thin films, the formation is merely suggested as possible. In fact, there is made little or no reference to moldability and processability of polymers which are considered to be most important in the precursor method.
(3) There is shown production of silazane polymers, for example, by reaction between chlorodisilanes and disilazanes in Japanese Patent Laid Open No. Sho 57-117532 by reaction between chlorosilanes and disilazanes in Japanese Patent Laid Open No. Sho 57-139124, by reaction between chlorodisilanes and ammonia in Japanese Patent Laid Open No. Sho 58-63725 and by reaction between trichlorosilane and disilazanes in Japanese Patent Laid Open No. Sho 60-135431 respectively. Moreover, disclosures are also made for the production of silazane polymers by addition of meal halides to chlorosilanes and disilazanes in U.S. Pat. No. 4,535,007 and by addition of metal halides to chlorodisilanes and disilazanes as disclosed in Japanese Patent Laid Open No. Sho 60-208331 respectively. It is stated in each of these references that the silazane polymers mentioned above may be converted to ceramic materials by pyrolysis. However, the ceramic yields for all the silazane polymers are, at most, 50 to 60 wt. %. Similar to the patent specification shown in (2) above, none of the above references describes, in detail, the moldability and processability of the polymers, which are most important in the precursor method. In particular, most of the references mention nothing for ceramic fibers in examples, or do not refer to strength of ceramic fibers in case where examples of ceramic fibers are shown. Only in Japanese Patent Laid Open No. Sho 60-208331, there is a description of strength, but ceramic fibers having such a low tensile strength as of 53 Kg/mm.sup.2 or 63 kg/mm.sup.2 are obtained.
(4) In Japanese Patent Laid Open No. Sho 60-226890, there is described a process of preparing silazane polymers which comprises reacting ammonia with an organic silicon compound of the formula, ##STR4## to obtain an ammonolysis product and subjecting the product to condensation by dehydrogenation with alkali metal or alkaline earth metal hydrides. It is stated that the polymers obtained in this process can be controlled in property depending on the degree of condensation by deprotonation and may take various forms of from oils to solids having no definite melting points. However, when a polymer melt is molded or processed to prepare, for example, a continuous fiber by melt spinning, it is necessary that the polymer have a certain degree of polymerization and be thermally stable. In the above process, however, the polymer obtained will be in the form of a solid which has not a melting point unless the polymerization is stopped on its way. In order to obtain a fusible polymer, the reaction time, reaction temperature, amounts of catalyst and solvent, etc. have to be controlled precisely but such a control may be very difficult and may not be usually reproducible. The polymers obtained by the process are not thermally stable with the disadvantage that gel-like substances are formed. In view of the above two problems, this process may not be considered to be suitable as an industrial process of manufacturing silazane polymers.
(5) Japanese Patent Laid-open No. Sho 60-228489 describes a process of preparing a silazane polymer which comprises producing a cyclic silazane from a compound of the formula: ##STR5## and monomethylamine, followed by reacting the cyclic silazane with ammonia. In this patent application, it is stated that the polymer is suitable as a material for chemical vapor deposition, but physical properties of the polymer are not described in detail. The ceramic yield is not indicated at all.
As will be apparent from the foregoing description, hitherto proposed silazane polymers, serving as preceramic materials, are not beneficial for industrial production. In addition, these polymers were found to be poor with respect to moldability and processability as precursors for ceramic fibers and poor in the ceramic yield as well. Ceramic products, e.g. ceramic fibers, obtained from the known preceramic polysilazane materials were found to have relatively poor physical properties such as strength, modulus of elasticity and the like.